Role of faults, nonlinear rheology, and viscosity structure in generating plates from instantaneous mantle flow models

Abstract

Concentrated strain within plate margins and a significant toroidal component in global plate motion are among the most fundamental features of plate tectonics. A significant proportion of strain in plate margins is accommodated through motion on major tectonic faults. The decoupling influence of faulted plate margins primarily results from history-dependent lithospheric deformation rather than from instantaneous stress-weakening rheologies. For instantaneous mantle flow models, we argue that faults should be treated as preexisting mechanical structures. With models incorporating preexisting faults, a power law rheology with an exponent of 3, and slab pull and ridge push forces, we demonstrate that nonlinear interaction between weak faults and this power law rheology produces plate-like motion. Our models show that in order to produce plate-like motion, the frictional stress on faults needs to be small and the asthenosphere viscosity should be much weaker than that of lithosphere. While both plateness and the ratio of toroidal to poloidal velocities are reduced with increasing fault coupling, the viscosity contrast between the lithosphere and asthenosphere only influences plateness. This shows that both diagnostics, plateness and the ratio of toroidal to poloidal velocities, are necessary to characterize plate motion. The models demonstrate that weak transform faults can guide plate motion. This guiding property of transform faults and the decoupling of thrust faults result in oblique subduction where the strike of subducted slabs is oblique to transform faults. Subducted slabs beneath a dipping fault produce oceanic trench and fore bulge topography and principal stresses consistent with subduction zone observations.